1. Technical Field
The present invention relates generally to electronic instrumentation and interfaces for computers. More particularly, the invention pertains to a circuit for obtaining a digital representation of the frequency of an input signal. In one specific application, the invention relates to a method and apparatus for digitally signaling the speed of an engine or turbine over a wide range of speed using a compact but high-resolution digital representation.
2. Description of the Related Art
It is well known to obtain a digital representation of the frequency of an input signal by counting the number of cycles in the input signal over a predetermined counting interval. Also, it is known to obtain a digital representation of the frequency of an input signal by counting the number of cycles of a reference signal occurring within one or more cycles of the input signal, and multiplying the known frequency of the reference signal by the ratio of the number of cycles of the input signal and the counted number of cycles in the reference signal.
Cording and Teager, U.S. Pat. No. 4,485,452 issued Nov. 27, 1984 and titled "Speed Measurement System" describes a method of measuring the speed of a rotating element over an extended range by counting both the cycles of an input signal from a speed sensor and the cycles of a reference signal over a counting interval that is asynchronous to the reference signal. The counted number of cycles of the input signal is divided by the counted number of cycles of the reference signal. The input signal is counted in a tooth counter, and the reference signal is counted in two counters including an interval counter and a vernier counter. The tooth counter is reset at the start of the counting interval, and is sampled or latched at the end of the counting interval. The vernier counter is reset at the start of each input signal cycle and is sampled or latched twice, when the tooth counter is reset and also when the tooth counter is latched. The interval counter is enabled for counting when the tooth counter is reset and is disabled for counting when the tooth counter is sampled or latched. The division takes place and all of the counters are reset in response to a maximum interval counter which counts a predetermined number of counting interval cycles. The latched value of the tooth counter is divided by additions and subtractions of the interval counter value and the latched values of the vernier counter to generate a value proportional to the speed of the rotating element.